1. Field of the Invention
The present invention relates to an image forming apparatus for fixing toner onto a recording sheet by application of heat.
2. Description of the Related Art
In conventional image forming apparatuses, a fixing heater is heated by using a commercial power source. Also, in recent years, as an energy saving measure, proposed is an image forming apparatus with an energy saving mode at which power supply to the fixing heater is suspended. In the image forming apparatus provided with the energy saving mode, once the apparatus enters the energy saving mode, power supply to the fixing heater is temporarily suspended, with the result that the temperature of the fixing heater falls below a predetermined temperature suitable for image fixation. Accordingly, a waiting time, i.e., a temperature-recovering time for raising the temperature of the fixing heater to the predetermined temperature suitable for image fixation is required to return the apparatus from the energy saving mode to a print mode.
In view of the above, there has been proposed an image forming apparatus with a shortened waiting time by maintaining the temperature of the fixing heater in the energy saving mode at a temperature slightly lower than the temperature of the fixing heater suitable for image fixation during printing.
Use of a large electric power enables to quickly raise the temperature of the fixing heater. However, in light of the fact that the commercial power source available in Japan is 1,500 W at maximum, with the voltage of 100V and the current of 15 A, the use of the large electric power is normally impossible. Japanese Unexamined Patent Publication No. 10-282821 discloses a heating device with an enhanced energy saving effect, or an image forming apparatus provided with the heating device, wherein a chargeable sub power source such as a capacitor or a battery is provided, the sub power source is charged by a commercial power source while the apparatus is in the energy saving mode, and a large electric current is allowed to flow through the fixing heater by using both the commercial power source and the sub power source at the time of returning the apparatus from the energy saving mode to the print mode to shorten the temperature-recovering time. In the arrangement, the enhanced energy saving effect is secured because the temperature-recovering time can be shortened even if the temperature of the fixing heater at the energy saving mode is set significantly lower than the temperature of the fixing heater suitable for image fixation.
Also, there is known an image forming apparatus designed such that an electric power is supplied through two power source outlets simultaneously in place of using the sub power source.
In the conventional image forming apparatus, when a long-time printing is conducted, there is likelihood that the temperature of the fixing heater may fall below a predetermined temperature suitable for image fixation during the long-time printing.
The following is a description on features about an electric dual layer capacitor, which is a suitable capacitor example as the sub power source, and a lithium ion battery, which is a suitable battery example as the sub power source.
The electric dual layer capacitor is chargeable and dischargeable with a large electric current, and has a relatively long useful life with durability of repeated charging and discharging operations in the order of, e.g., about several hundred thousand times, yet with a drawback that the voltage rapidly falls. In view of this, it is necessary to make the capacity of the capacitor large in order to supply an electric power for a long time, which is costly. The time required for charging the electric dual layer capacitor is, e.g. several ten seconds.
The lithium ion battery has a merit that voltage fall is significantly small until an end stage of discharging. Therefore, as compared with the electric dual layer capacitor, long-time power supply is realized with a less cost. The lithium ion battery, however, has a demerit that its useful life may be shortened if charging and discharging operations are repeated with a large electric current. In the case where a halogen heater, which is used as the fixing heater, is energized from a state where the temperature of the halogen heater is lowered to ambient temperature, an in-rush current, e.g., a large current of about ten times as large as a rated current may rapidly flow through the halogen heater. If the lithium ion battery is used in the above state, the useful life of the lithium ion battery may be shortened to such an extent that a possible maximal number of repeatedly performing charging and discharging operations may be decreased to about 500 to 1,000 times. The time required for charging the lithium ion battery is about 10 minutes, for instance.
It is desirable to allow a large current to flow into the fixing heater for a long time from the point of time when the apparatus is recovered from the energy saving mode to the print mode until the time of completion of printing, with use of a sub power source as well as a main power source during the printing, in order to prevent the temperature of the fixing heater in the printing operation from falling below a predetermined temperature suitable for image fixation, and to keep the temperature of the fixing heater at the energy saving mode as low as possible, yet shortening a temperature-recovering time required for the apparatus to recover from the energy saving mode to the print mode. However, in light of the demerit of the electric dual layer capacitor that the voltage thereof rapidly falls once a discharging operation starts, in use of the electric dual layer capacitor as the sub power source, a measure is necessary to provide the electric dual layer capacitor with durability of long-time use. The measure is provided by making the capacity of the electric dual layer capacitor large, which, however, is costly.
The lithium ion battery has the demerit that its useful life may be shortened by repeated charging and discharging operations with a large current. In use of a halogen heater in a fixing roller, a large current may flow through the halogen heater in the case where the halogen heater is energized from the state where its temperature is lowered close to the ambient temperature. Accordingly, use of the lithium ion battery as the sub power source in order to rapidly heat the fixing heater, whose temperature has been lowered close to the ambient temperature, may shorten the useful life of the lithium ion battery.